Thin film deposition system

ABSTRACT

A METHOD AND APPARATUS FOR CHEMICAL VAPOR PHASE DEPOSITION OF SEMICONDUCTOR MATERIALS ON A SEMICONDUCTOR SUBSTRATE PROVIDES THE CAPABILITY FOR DEPOSITING CONTINUOUS UNIFROM FILMS OF SEMICONDUCTOR MATERIAL LESS THAN 1 MICRON THICK. A SEMICONDUCTOR SUBSTRATE ONTO WHICH A THIN FILM IS TO BE DEPOSITED IS POSITIONED AND LOCATED IN A ZONE OF NO FLOW OR STAGNATION IN A RECTOR ENCLOSURE. GASEOUS REACTANTS ARE CONTINUOUSLY SUPPLIED TO THE REACTOR AND ARE CONTINUOUSLY WITHDRAWN FROM THE REACTOR ALONG WITH BY-PRODUCTS OF THE DEPOSITION REACTION. THE REACTANTS CONTACT THE SUBSTRATE BY MOVING THROUGH THE AREA OF STAGNATION NOT BY MACROSOPIC VAPOR FLOWBUT BY MOLECULAR DIFFUSION FROM THE AREA OF FLOW THROUGH THE STAGNANT AREA TO THE SEMICONDUCTOR SUBSTRATE. THE AREA OF STAGNATION CAN ALSO BE PROVIDED WITH A BACKWASH OR FLUSHING MEANS WHICH PROVIDES A POSITION MECHANISM FOR BEGINNING AND STOPPING DIFFUSION INTO THE STAGNANT   AREA, THUS CREATING A DEPOSITION PERIOD OF VERY DEFINITE LENGTH.

Oct. 31, 1972 w. w. GARTMAN ET 3,701,582

THIN FILM DEPOSITION SYSTEM Filed July 2, 1970 2 Sheets-Sheet 1 (D KO INVENTORS'.

HAROLD A. ALLEN JAMES G. EVANS 7 WILLIAM WGARTMAN Oct. 31, 1972 w. w. GARTMAN ET AL 3,701,682

THIN FILM DEPOSITION SYSTEM Filed July 2, 1970 2 Sheets-Sheet 2 6 He H 7 //4 INYENTORS:

HAROLD A. ALLEN JAMES 6. EVANS WILL/AM W GARTMAN j 'United States Patent Oflice 3,701,682 Patented Oct. 31, 1972 3,701,682 THIN FILM DEPOSITION SYSTEM William W. Gartman and James G. Evans, Plano, and

Harold A. Allen, Dallas, Tex., assignors to Texas Instruments Incorporated, Dallas, Tex.

Filed July 2, 1970, Ser. No. 52,030 Int. Cl. B01j 17/30 US. Cl. 117-201 12 Claims ABSTRACT OF THE DISCLOSURE A method and apparatus for chemical vapor phase deposition of semiconductor materials on a semiconductor substrate provides the capability for depositing continuous uniform films of semiconductor material less than 1 micron thick. A semiconductor substrate onto which a thin film is to be deposited is positioned and located in a zone of no flow or stagnation in a reactor enclosure. Gaseous reactants are continuously supplied to the reactor and are continuously withdrawn from the reactor along with by-products of the deposition reaction. The reactants contact the substrate by moving through the area of stagnation not by macroscopic vapor flow but by molecular diffusion from the area of flow through the stagnant area to the semiconductor substrate. The area of stagnation can also be provided with a backwash or flushing means which provides a positive mechanism for beginning and stopping diffusion into the stagnant area, thus creating a deposition period of very definite length.

This invention relates to formation of a thin film of semiconductor material and more specifically relates to chemical vapor deposition of thin films of semiconductor material on a semiconductor substrate or surface, which films are uniform, continuous and generally have a thickness of less than 1 micron.

Various compositions composed of semiconductor materials from groups II-I-A, IV-A and V-A of the Periodic Table as it appears in the Handbook of Chemistry and Physics, 45th edition, 1964, Chemical Rubber Publishing Company, Cleveland, Ohio, have become very useful in the semiconductor industry. Methods for forming single crystalline layers by epitaxial deposition from the chemical vapor phase are known to the art. These semiconductor compositions have heretofore been deposited in thicknesses usually in excess of 5 microns, ranging up to or more mils. It is desirable, depending upon the various applications, to deposit single elements, for example, silicon, or to deposit compounds, binary, or tertiary alloys or pseudo-alloys of these elements.

Recent developments in the opto-electronic industry have led to the need for a technique to epitaxially deposit layers of group III-A-V-A binary and ternary alloys, for example, gallium indium arsenide, which are less than 1 micron thick. Commonly these alloys are chemically formed from the vapor phase by the reaction:

Various attempts have been made to deposit such thin layered compositions but all have resulted in failure in that the layers produced were discontinuous, uneven, too thick, or could not be grown at all.

These previous attempts at depositing or growing a thin layer of such binary alloys used conventional reactor designs. The unsuccessful procedures utilize fast growth rate reactions for very short deposition times.

These deposits have either been greater than 1 micron thick or have been discontinuous across the substrate. Such conventional reactor designs are self-limiting in minimum deposit thickness because the reaction vessels themselves act as large reservoirs, resulting in after growth, i.e., reaction and deposition taking place after reactant flow to the reactor has ceased, such after growth increases the thickness of the deposited layer beyond 1 micron. Another problem associated with these fast growth rate, short duration reactor systems is the type of surface nucleation which results. In such systems deposition is started at an isolated island. This island grows more rapidly than does the surrounding area. At desired deposition levels of less than 1 micron, a continuous film is prevented from forming until the layer is several microns in thickness. Even at this point, the layer is of uneven thickness.

Other attempts have been made to slow down the growth rate by decreasing the flow of gas through the reactor. These alone have not been wholly satisfactory for a number of reasons including actual physical limitation in the size of the reactor, tubing size limitations, and available reactants. The deposition rates can be slowed to produce continuous films of less than 1 micron in thickness by quantitatively diluting the reactant gases with a carrier gas. This procedure, however, become impractical from a technical or engineering standpoint.

It is, therefore, desirable to possess a system in which a low deposition rate resulting in continuous, even layers of semiconductor material less than 1 micron in thickness can be achieved utilizing with conventional flow rates and slightly modified, conventional reactor designs. This desire has been achieved by slight modification of conventional chemical vapor phase deposition, epitaxial reactor systems. The method and apparatus of the present invention provide a means by which reproducible layers of semiconductor material less than 1 micron thick can be deposited.

This invention, therefore, provides a chemical vapor phase deposition reactor comprising a member defining a flow path for gaseous components and a region of fluid stagnation, and means for positioning a substrate in the region of stagnation. More specifically, the in-, vention provides an apparatus for depositing thin films of a semiconductor material comprising, a first reactor housing, means for admitting gaseous reactants to the interior of the first housing, a second housing having means therein for holding a deposition substrate, the second housing communicating through an opening therein with the first housing, and means for withdrawing gaseous components including unused reactants and byproducts.

The invention also provides a method for depositing layers of semiconductor material comprising passing a gaseous stream of reactants past a stagnant zone, allowing a portion of the gaseous stream to transfer into said stagnant zone, and contacting a substrate positioned in the stagnant zone with the reactants which have transferred into the zone, thereby reacting the reactants and depositing a semiconductor material on the substrate. More particularly, the method comprises supplying a reactant to a first zone in a reactor, diffusing the reactant into a second zone in the reactor, the second zone being a region of stagnation containing a deposition surface, depositing a thin film of semiconductor material on the surface by reacting the diffused reactant near the surface producing a semiconductor material which deposits on the surface, and withdrawing a composite stream including a reaction product and excess reactant from the reactor.

The present invention will be described in relation to a preferred embodiment and an alternate embodiment thereof. Although'a single variant of the preferred embodiment is disclosed, it will be apparent to those of ordinary skill in the art that a number of alterations, substitutions, modifications and variations can be made upon 3 these embodiments and yet remain within the intent and concept of the present invention. Therefore, the invention is intended only to be limited and defined by the appended claims.

A better understanding of the present invention will be derived by reference to the ensuing specification to be read in conjunction with the accompanying drawings wherein:

FIG. 1 is a schematic representation of a preferred reactor system of the present invention; and,

FIG. 2 is an alternate embodiment of a reactor system of the: present invention.

Referring now to FIG. 1, a large quartz tube serves as the primary reactor enclosure. The tube 10 has tapered ends onto which are slidably disposed caps 12 and 14. The interfaces between the caps 12 and 14 and the tube 10 are. usually lubricated with a suitable sealant which is temperature resistant. Surrounding the left hand portion of the reactor is a first bank of heaters 16, These heaters can be of the electric resistance type, RF powered, or any other suitable constant temperature, controllable heating means. A second bank of heaters 18 surrounds the right hand portion of the quartz tube 10. The heaters 18 can be independently controlled from the heaters 16 to maintain a different temperature in the left portion of the reactor from that maintained in the right portion of the reactor. The reactor and heaters are insulated by a layer of quartz wool 19, or other suitable insulation.

Attached to the interior of right cap 12 is a second tube 20 of smaller diameter than the primary reactor enclosure 10. One end 22 is bonded to cap 12 by a suitable gas tight seal. For example, if tube 20 and cap 12 are quartz, the tube 20 can be melt bonded to the cap 12. The other end 24 of the tube 20 is open and faces toward the other end of the reactor. Mounted inside the smaller tube 20 is a substrate support 26, on which is shown positioned a semiconductor substrate 28 onto which a thin layer of semiconductor material is to be deposited.

Referring to the supply system for the reactor, a source of hydrogen is connected to three-way valve 30. A source of helium is also connected to the valve 30. A common conduit 32 supplies a series of valves and rotameters. The valve 30 can be utilized to direct a flow of hydrogen or helium to conduit 32 as desired, depending upon the particular time to which the sequence of operating the deposition process has progressed. A first valve and rotameter arrangement 34 controls the flow of gas into conduit 36 which extends through the cap 12 into the interior of the small diameter tube 20. A second rotameter 38 supplies gas to a conduit 40 which is inserted through cap 14 to provide a flow of gas to the entire interior of the reactor structure.

In conjunction with the method of operating the apparatus of FIG. 1, the deposition of gallium indium arsenide has been chosen. Arsine, a first reactant, is supplied to three-way valve 40. This valve 40 is also suppliedwith either helium or hydrogen from conduit 32. The outlet from valve 40 runs to rotameter 42, through three-way valve 44 and into conduit 46. Likewise, the gas from conduit 32 flows through rotameter 48 and combines with the arsine from three-way valve 44 in conduit 46. Conduit 46 is connected to conduit 50 which in turn extends through cap 14 into the interior of the large diameter reactor tube 10.

In a similar manner, hydrochloric acid is supplied to threeeway valve 52. Likewise, gas from conduit 32 is also supplied to three-way valve 52. The outlet from the valve 52 goes to rotameter 54 through three-way valve 56 and into conduit 58. Gas from conduit 32 enters into and is controlled by the valve and rotameter arrangement 60, the output from which combines with the hydrochloric acid in conduit 58. Conduit 58 connects with conduit 62 which extends through cap 14 into the interior of large reactor tube 10. An enlarged portion 64 of the conduit 62 also referred to as a boat is provided for the placement and storage of reactive components, for example, predetermined amounts of gallium and indium. The hydrochloric acid diluted with a carrier gas flowing into conduit 68 and through the enlarged portion 64 thus combines with the gallium and indium to provide gallium chloride and indium chloride. These products along with the arsine form the reactants from which a thin layer of gallium indium arsenide will be deposited on substrate 28.

Gas is also supplied to an additional rotameter 66 which in turn supplies conduit 68 extending through cap 14 into the interior of the large reactor tube 10. Conduit 68 also contains an enlarged portion or boat 70 in which is contained any desired impurity or dopant, for example, zinc. The dopant in vaporous form will combine with the other reactants to provide a final deposited layer of desired and predetermined composition.

The reactant gases in excess of those needed to deposit the desired layer thickness, the by-products of the deposition reaction, the excess hydrochloric acid and the carrier gases, are removed through conduit 72 which extends through cap 12. Conduit 72 empties into a bubbler unit 74 from which it enters conduit 76 and is vented to atmosphere or to a suitable burn-off device.

In operation, the reactor system is first flushed with helium to remove all contaminate gases, especially oxygen. Valve 30 is initially positioned so as to admit helium from a source into conduit 32. The valve and rotameter arrangements 34, 38, 48, 42, 60, 54, and 66 are adjusted and calibrated to maintain the desired flow rates. Initially, valve 40 is positioned to admit helium to the rotameter 42. Also initially, valve 52 is positioned to admit helium to rotameter 54. Valves 44 and 56 are also positioned to allow the helium to pass into conduits 46 and 58 from rotameters 42 and 56, respectively. Removal of the oxygen thus will also prevent the occurrence of an explosive mixture when hydrogen, the preferred carrier gas, is admitted at a later stage in the process.

After the reactor has been purged of all contaminant gases, three-way valve 30 is positioned to admit hydrogen from the source of hydrogen into conduit 32, thus stopping the flow of helium. Subsequently, valves 44 and 56 are turned to an exhaust position thus allowing hydrogen to escape to atmosphere or preferably a suitable burn-off device. Thereafter, valves 40 and 52 are positioned to admit arsine and hydrochloric acid to rotameter arrangements 42 and 54, respectively. At this point in the process, hydrogen is flowing through rotameters 34, 38, 48, 60 and 66. Arsine is flowing through rotameter 42 and is exhausted through valve 44. Likewise, hydrochloric acid is flowing through rotameter 54 and is exhausted through valve 56.

After the helium in the reactor tube '10 and reactor tube 20 has been completely displaced by a hydrogen atmosphere gas, heaters 16 and 18 can be activated to bring the reactor up to appropriate temperatures. As previously mentioned, heater 16 can be controlled independently of heater 18. Usually, heater 16 which surrounds the zone containing the inlet for the reactant vapors and the dopant vapor is maintained at a temperature higher than that within the zone heated by heater 18, the deposition zone. It will be noted that the substrate support 26 and substrate 28 are positioned within the zone heated by heater 18. When the two zones in the reactor have reached equilibrium at their respective, preselected temperatures, valves 44 and 56 are positioned to allow the introduction of arsine and hydrochloric acid into conduits 46 and 58, respectively, and thus into the zone surrounded by heater 16. A flow pattern is established toward opening 24 of the small diameter of tube 20. However, since the tube 20 has no outlet, a stable flow pattern is formed around the exterior of the tube 20 and into the outlet conduit 72. This equilibrium flow pattern is established quickly.

Without being bound by theory, it is believed that initially when the flow rates are being established turbulence may occur in the seed crystal tube, especially in the embodiment of Example I. However, after the flow rates have been established and are constant, the system will reach an equilibrium flow where there is substantially no flow in the deposition zone tube. Therefore, it is believed that diffusion from the mouth or opening of the tube up toward the substrate is what materially contributes to the low deposition rate. Also, products such as hydrochloric acid in the reaction set forth above, are formed at the surface of the substrate. These products must diffuse out of the deposition zone before more material can be deposited, since the reaction is an equilibrium reaction. The vaporous reactants then do not actually macroscopically flow in streams past the substrate 28. Instead, molecular diffusion or mass transfer through the opening 24 down the tube 20 toward the substrate 28 occurs.

However, diffusion will not occur until desired by utilization of the backwash or flushing flow of hydrogen obtained via conduit 36. This flow of hydrogen prevents any diffusion into the tube 20 until the rotameter and valve arrangement 34 is turned off. Thus, when the flow from conduit 36 is stopped, diffusion can begin to take place from the reactant compositions present near the mouth or opening 24 of the tube 20. After a desired deposition time, the flush or backwashing flow from conduit 36 can again be commenced by turning on the rotameter and valve arrangement 34. By doing so, the diffusion down tube 20 toward substrate 28 will be stopped. Accordingly, deposition will cease.

After hydrogen flow into the deposition zone has again been started, the valves 44 and 56 can be positioned to exhaust, the valves 40 and 52 positioned to admit hydrogen to the rotameters '42 and 54 and this, of course, will stop the flow of arsine and hydrochloric acid to the reactor. Thereafter, valve 30 can be repositioned to admit helium from its supply to the conduit 32. Heaters 16 and 18 can also be deenergized after hydrogen flow through conduit 36 is begun. When the reactor is completely emptied of hydrogen, the cap 12 can be removed and the substrate 28 removed from the tube 20. By following this procedure, a film of less than 1 micron will be deposited on the substrate 28.

Referring now to FIG. 2, an alternate embodiment of the present invention is illustrated. In this particular reactor design, the reactor is shown in a T configuration. A vertical quartz tube '80 has tapered ends onto which are fitted caps 82 and 84. The caps and ends of tube 80 are sealed at their interfaces with a heat resistant sealant. About midway along the length of the reactor tube 80, a horizontal tube 86 opens into and is fused to the vertical tube 80. The end of horizontal tube 86 is also tapered for engagement with cap 88. The interface between cap 88 and the tapered ends of tube 86 are likewise sealed with a heat resistant temporary sealant.

A first heating element 90 is positioned about the upper portion of the vertical reactor tube 80 and extends to a position slightly below the region where horizontal tube 86 joins vertical tube 80. A second two-piece heating element 92, which can be independently controlled from heating element 90, is positioned below heating element 90 and is connected in series with a second heating element 94 surrounding the inner portion of horizontal tube 86. The heating elements are then surrounded by a layer 96 of quartz wool or other suitable insulation.

This embodiment of the reactor will be described in conjunction with the deposition of gallium arsenide on a semiconductor substrate. Gallium metal 98 is contained in a boat 100 connected to a conduit 102. The conduit 102 in turn extends through the top cap 82. The boat 100 has an opening 104 through which gallium chloride can escape. Positioned below the boat 100 is a fused silica frit 106 which is attached to the interior walls of the vertical tube 80 by a circular bracket 108. Any mass transfer occurring from above the frit 106 to below the frit 106 must pass through the frit itself. Thus the frit 106 provides a means for slowing down mass transfer into the deposition zone wherein the substrate is positioned. i.e., the frit provides resistance to but not complete blockage of mass transfer. A substrate 110 is positioned in the lower portion of the vertical tube below the frit 106 on a suitable pedestal 112. The pedestal is attached to the cap 84 for easy removal and insertion of the substrate 110.

Similar to the preferred embodiment illustrated in FIG. 1, sources of helium and hydrogen are provided and are connected to the input ports of a three-way valve 114. The three-way valve is connected to a conduit 116 from which a series of rotameters 118, 120, 122, 124, 126 and 128 are fed. The fiow through the rotameters is controlled by a valve associated therewith. Arsine, a reactant for the deposition of gallium arsenide, is supplied from a source to an input port of a three-way valve 130. The other input port is connected to conduit 116. The exhaust port of valve 130 is connected to rotameter 124. A threeway valve 132 is provided at the outlet from rotameter 124 to exhaust the output from that rotameter when it is not desired that it enter the reaction system. The threeway valve 132 is shown in its open position which will allow the combination of the arsine from rotameter 124 with hydrogen from rotameter 122. Thus, hydrogen provides a carrier stream for the arsine while also providing a means for diluting the arsine. The combined arsine and hydrogen then travels through conduit 134 and into conduit 136. The latter conduit extends through cap 82 and downwardly to a point where it can combine with the vaporous reactant issuing from opening 104.

Similarly, hydrogen is supplied to rotameter 126 and is combined with the vaporous stream from rotameter 128 in conduit 138. Hydrochloric acid is supplied to an inlet port of three-way valve 140. Hydrogen can also be supplied to the other inlet port of three-way valve from conduit 116. The outlet of three-way valve 140 is connected to rotameter 128 which in turn is connected to an inlet port of three-way valve 142. Similarly, to three-way valve 132, three-way valve 142 can be positioned to exhaust hydrochloric acid if it is not wanted in the reaction system. The outlet port of three-way valve 142 is usually connected, however, to conduit 138 where it is combined with the hydrogen output of rotameter 126. Conduit 138 connects to conduit 102 which extends into the boat 100. The hydrochloric acid vapors there react with the gallium 98 to form gallium chloride vapor. The gallium chloride vapors, issuing from the opening 104 in the boat, and the arsine vapors, issuing from conduit 136, are mixed by the turbulent fiow pattern within the reactor and are directed primarily toward the intersection of vertical tube 80 and horizontal tube 86.

Cap 88 on horizontal tube 86 is provided with an exhaust conduit 140 which extends below the surface of a liquid in a bubbler unit 142. In addition, a second exhaust conduit 144 extends through cap 84 on the lower portion of the vertical tube 80. This exhaust conduit 144 is provided With a valve 146 which can be adjusted as will be explained hereinafter. The conduit 144 extends from cap 84 below the level of liquid in the bubbler unit 142. The exhaust gases, which include excess reactants and byproducts of the reaction taking place in the reactor, are directed from the bubbler unit through conduit 148 preferably to a burn-off device.

The lower portion of the vertical tube 80 is also provided with a flush or backwashing stream of gas via conduit 150 extending through cap 84. Conduit 150 is supplied by a conduit 152 from rotameter 118.

During operation of the apparatus illustrated in FIG. 2, the excess arsine and gallium chloride vapors are primarily exhausted through the horizontal tube 86 into the bubbler unit 142. When the reaction system is started, flow is allowed through conduit 150 into the lower portion of the vertical tube 80. This prevents any diffusion or passage of gas into the lower deposition zone wherein the substrate 110 is positioned. After the reaction system has reached temperature and flow equilibrium, the flow of hydrogen from conduit 150 is stopped by turning ofi? the rotameter and valve arrangement 118. This will allow diffusion through the porous frit 106 into the deposition zone. When the gallium chloride and arsine contact the surface of the substrate 110, gallium arsenide is formed and is deposited in a thin layer on the substrate 110. Good mass transfer control can be obtained utilizing the frit which separates the deposition zone from the zone in which the reactants are admitted to the reactor.

However, the deposition rates obtained by the above procedure are very slow and can be increased by opening valve 146. By doing so, a portion of the exhaust flow is then allowed to pass through the bottom portion of the vertical tube 80, out conduit 144, and into 'bubbler unit 142. As can readily be seen, the deposition rates will increase. By utilizingthis technique in conjunction with the backwash flow from conduit 150, precise control over the deposition rate can be obtained.

The following examples are included to illustrate the unique capabilities of the present invention. They are intended only to be exemplary and not to be delimitative in any manner. All percentages used herein are weight percentages.

EXAMPLE I An apparatus of configuration similar to that shown in FIG. 1 is constructed from a quartz tube 30 inches long and about 60 millimeters in diameter. Reference numerals will be utilized where appropriate. The tube has standard tapered joints fitted with quartz caps, through which quarter inch inlet and outlet conduits are inserted. The tubing leading fromthe inlet and outlet conduits to the rotameters is one-eighth inch stainless steel tubing. standard resistant heaters are wound about the appropriate areas of the quartz reactor tube and connected to an energy source through a switch. A first resistance heater 16 is wound about the inlet portion of the quartz tube reactor. A second heater 18 surrounds the other end of the reactor containing the opening to the small diameter quartz tube at the substrate suppor. The smaller tube containing the substrate support is about 25 mm. in diameter and about 10 inches long. Quartz wool is wrapped about the resistance heaters to insulate them. Thermocouples can also be inserted through suitable sealed openings in the quartz tube so that good temperature control can be obtained.

25.77 grams of indium and 2.74 grams of gallium are placed in the boat 64. A gallium arsenidesubstrate or seed crystal which has been mechanically lapped, chemically polished and cleaned by conventional methods to expose the lattice oriented to the 100 plane is then placed on the substrate support. The crystal is 20 millimeters in diameter and approximately 20 mils thick. The chemical polishing solution utilized is a 0.2% aqueous solution of sodium hypochlorite. The seed crystal is chemically cleaned or etched in an 8-1-1 weight ratio of sulfuric acid, hydrogen peroxide and Water, respectively, for about 10 minutes prior to placing it in the reactor.

The system is then started by admitting helium to all of the rotameters. The rotameters are all adjusted to the following settings: the backflush rotameter 34, 400 cc./ min.; the carrier flush rotameter 38, 120 cc./min.; the

arsine carrier rotameter 48, 70 cc./min.; the arsine rotameter 42, 60 cc./min. (4 mole percent arsine in hydrogen); (the three-way valve 40 is first positioned to admit helium to the arsine rotameter); the hydrochloric acid carrier rotameter 60, 60 cc./min.; the hydrochloric acid rotameter 54, .20 cc./min. (5 mole percent in hydrogen); (the three-way valve 52 is first positioned to admit helium to the hydrochloric acid rotameter); the dopant rotameter 66, 20 cc./min. The system is flushed with helium for about minutes. The two electric resistance heaters 16 and 18 are then energized to heat the reactant feed zone to about 900 C. and the deposition or substrate zone to about 750 C. These temperatures are maintained throughout the remainder of the reaction. After maintaining the helium flow for a sufficient amount of time to assure substantially complete removal of oxygen, the flow of helium is stopped and the flow of hydrogen initiated by repositioning valve 30. After the temperature in the reactor has stabilized, the hydrochloric acid and arsine valves, 40 and 52 respectively, are positioned to initiate the flow of arsine and hydrochloric acid into the reactor. The exhaust valves 44 and 56 are positioned to admit arsine and hydrochloric acid to the reactor. The backwash or flush flow is maintained for about 15 minutes after initiating the flow of reactants into the system. Thereafter, the backwash flow is terminated.

Deposition on the substrate occurs substantially through diffusion into tube 20 for about 10 minutes, at which point the backwash flow is again started at a rate of about 400 cc./min. The exhaust valves 44 and 56 are then turned to their exhaust position and the flow of hydrochloric acid and arsine is terminated. The electric resistance heaters are then deenergized and the reactor allowed to cool to room temperature. During this cooling process, the flow of hydrogen is terminated and helium is admitted to the reactor to purge it'of hydrogen. After the substrate has cooled to room temperature, it is removed, cleaved along a diameter thereof, and magnified under an electron microscope. The layer is uniform over the surface of the substrate and has a thickness of about 0.5 micron. The film is analyzed. It contains a molecular ratio of 0.72 to 0.28 to 1.0 of gallium, indium and arsenic, respectively.

EXAMPLE II A chemical vapor deposition reactor similar to that illustrated in FIG. 2 is constructed. Reference numerals will be used where appropriate. The vertical tube is composed of 39 millimeter diameter quartz tubing having tapered ends thereon capable of accepting standard end caps. The vertical tube is 17 inches in height excluding the tapered ends. The horizontal tube 86 is connected about 1 inch above the center of the vertical tube. The horizontal tube has a tapered end thereon and is 6 /2 inches long excluding the tapered end 30 millimeters in diameter. A fused silica frit 106 is positioned 1 inch below the center of the vertical tube. An exhaust conduit is fitted onto an end cap 88. The end cap is sealed to the tapered end of the tubing with conventional heat resistant sealants. The hydrochloric acid, arsine and primary hydrogen flush inlet conduits are connected to the top cap 82. A boat for holding gallium is connected to the bottom of the hydrochloric acid conduit. A substrate support is connected to the bottom cap 84 so that the substrate will be positioned about 1 inch below the fused silica frit when the bottom cap is in place. Additionally, a backflush conduit is connected to the bottom cap. The bottom cap is sealed onto the bottom portion of the vertical tube with a conventional heat resistant sealant.

15 grams of gallium are placed in the boat. The top cap is then positioned and sealed onto the top portion of the vertical tube. Similar to the previous example, the rotameters, all of the conduits, and the reactor are flushed with helium to purge all oxygen and other contaminants from the system. The rotameter valves are adjusted for the following flow rates: the hydrochloric acid rotameter 128, 6 cc./min.; hydrochloric acid carrier rotameter 12.6, 35 cc./min.; arsine rotameter 124, 67.2 cc./min. (4 mole percent arsine in hydrogen); arsine carrier rotameter 122, 13 cc./min.; feed flush rotameter 120, 13 cc./min.; backflush rotameter 118, none.

The feed zone temperature is maintained at 885 C. by heater 90. The deposition zone and series zone along the horizontal tube is maintained at 835 C. by heater 92/94. After the reactor has been flushed with helium, the carrier gas flow to conduit 116 is switched to hydrogen. When the reactor has reached an equilibrium temperature, the flow of arsine and hydrochloric acid is begun by switching the valves 130 and 140 to receive hydrochloric acid and arsine, respectively. No backfiush flow is used in this example.

After allowing the arsine and hydrochloric acid to flow for about 25 minutes, the flows are stopped, the heaters deenergized and the reactor is again purged with helium. The substrate is removed after the reactor has cooled. (The substrate is /2 inch in diameter and 20 mils thick.) The substrate is cleaved along a diameter thereof and placed under an electron microscope. It is observed that the film is continuous and even. Upon measurement, the film is found to be about 0.5 micron thick. The film is an alloy composed of gallium arsenide.

EXAMPLE III A run similar to that of Example II is made. The flow rates are: 0.3 cc./min., hydrochloric acid; 3.0 cc./min., hydrogen carrier for the hydrochloric acid; 65.0 cc./min., arsine (4 mole percent arsine in hydrogen); 3.0 cc./min., hydrogen carrier for the arsine; and 13 cc./min., primary feed hydrogen flush (rotameter 120). The weight of gallium in the boat is 20 grams. The feed zone is maintained at 880 C. and the deposition zone at 845 C.

In this run, the fused silica frit is removed. The deposition substrate is a /2 inch diameter, 20 mil thick substrate of sapphire (A1 No backfiush is utilized during start up. The valve in the auxiliary exhaust line connected to the bottom of the vertical reactor is opened slightly to permit an exhaust flow of about 10 cc./min. The reactor is allowed to operate at equilibrium for 25 minutes after the arsine and hydrochloric acid flow are started. At the end of 25 minutes the hydrogen backflush into the deposition zone from the bottom of the vertical reactor is started at the rate of about 80 cc./min. Simultaneously, the auxiliary exhaust flow from the bottom of the reactor is stopped. The reactor is thereafter flushed with helium and allowed to cool.

After cooling to room temperature, the substrate is removed and examined under an electron microscope. The coating is observed to be uniform and continuous over the deposition surface. Upon measurement, the thickness of the gallium arsenide film thus deposited is found to be 0.2 micron.

It is believed that a number of parameters will affect the deposition rate of a semiconductor material through utilization of the present invention. Among these are the dimensions of the tube surrounding the substrate, the temperature gradient in the diffusion region, the diffusion coefiicients of the reactant species, the amount of dopant in the reactant composition, the ratio of the different reactants, the exposed lattice orientation of the substrate, the substrate temperature during deposition, and the concentration of the active species will all affect the deposition or growth rate on the seed crystal. It is believed the present invention combines all of these different parameters to provide a deposition rate resulting in a uniform continuous semiconductor film. This result has heretofore been unachievable.

As illustrated in the foregoing examples, these various parameters can be manipulated to achieve a deposition rate which is slow enough to allow reproducible control of deposit thicknesses of less than 1 micron. It is also to be recognized that the reactor system of the present invention can be utilized to produce thin films of both monocrystalline and polycrystalline semiconductor materials. The foregoing examples illustrate epitaxial monocrystalline deposition. The present invention can be utilized to reproducibly control such thin layer deposits from less than 0.2 micron up to and including more than 1.0 micron.

Therefore, what is claimed is:

1. A method for the epitaxial growth of semiconductor material on a compatible substrate comprising:

establishing a flowing stream of reactants comprising a decomposable semiconductor compound;

establishing a stagnant zone so positioned as to inhibit the flow of said reactant stream therethrough but in communication with said reactant stream thereby allowing diffusionlike transport of said reactants into said stagnant zone;

placing said substrate within said stagnant zone; and

heating said stagnant zone and said substrate to a temperature sufiiciently high to cause a deposition and growth of semiconductor on said substrate.

2. The method of claim 1 comprising:

passing a relatively inert gaseous material through said stagnant zone at preselected times to prevent reactive transfer into said zone.

3. The method of claim 1 further comprising:

establishing restricted exhaust from said stagnant zone thereby to draw a minor portion of said reactant stream through said stagnant zone and to increase the deposition rate.

4. The method of claim 3 comprising:

providing flow resistance between said reactant stream and said stagnant zone.

5. The method of claim 3 wherein the reactant stream is comprised of a semiconductor material selected from groups III-A, IV-A, V-A and mixtures thereof.

6. The method of claim 5 wherein the reactant stream comprises a composition from group III-A and a composition from group V-A. I

7. The method of claim 1 wherein the reactant stream comprises a gallium composition and an arsenic composition.

8. The method of claim 7 wherein the reactant stream comprises a gallium halide and arsine.

9. The method of claim 1 wherein the reactant stream is comprised of a semiconductor material selected from groups III-A, IV-A, V-A and mixtures thereof.

10. The method of claim 9 wherein the reactant stream comprises a composition from group III-A and a composition from group V-A.

11. The method of claim 10 wherein the reactant stream comprises a gallium composition and an arsenic composition 12. The method of claim 11 wherein the reactant stream comprises a gallium halide and arsine.

References Cited I UNITED STATES PATENTS 3,370,995 2/1968 Lowery et al 1l7106 X 3,392,066 7/1968 McDermott et a1. 148-175 FOREIGN PATENTS 756,284 4/1967 Canada 117106 X OTHER REFERENCES IBM Tech. Disc. Bull. pp. 525 and 526, vol. 9, No. 5, October 1966.

RALPH S. KENDALL, Primary Examiner US. Cl. X.R. 

